Air quality is a rising concern in many parts of the world. One air quality parameter of heightened interest is the level or amount of fine particulate matter. Air filters remove particulate contaminants from a moving gas stream, and specifically from air streams. Some exemplary contaminants include dust, lint, smoke, pollen, and/or dander. For instance, in conventional Heating, Ventilating, and Air Conditioning (HVAC) Systems, a filter is inserted into the air stream to remove contaminants from the air stream. The filtered air is then recirculated throughout the building.
Typically, in an HVAC system, an air filter is located next to the furnace or air conditioning system air handler (which typically contains a fan and fan motor). The air handler typically pulls air from the house or building through a “return” duct system and then blows the air through the heating or cooling system and back into the house or building through the duct system. The air filter is typically located at the point where the return duct enters the air handler.
Conventional air filters generally include filter media constructed from a porous material, such as open cell foam or a non-woven web of materials such as fiberglass, polypropylene, etc. As a stream of air is induced to flow through the air filter, particulate contaminants are entrapped or separated from the air stream and accumulate on the filter media. After a period of use in this manner, the accumulated contaminants begin to interfere with the flow of the air stream through the filter media. An increased pressure drop across the filter impedes air flow through the air filter and can potentially reduce the effectiveness of the entire HVAC system. To avoid this result, air filters be removed and either cleaned or replaced with a new air filter. However, typically the air filter is hidden from view during normal use, making it difficult to determine the condition of the filter media without removing the air filter from the HVAC system for visual inspection. Air filter removal can be challenging and can sometimes damage the air filter.
Devices have been provided that generate a change signal to alert a user to the clogged condition of the filter media and to indicate the need for inspection, cleaning, or replacement of the air filter. Such devices include simple pressure sensors that measure the pressure on the downstream side of the filter and generate a signal if the pressure drops below a predetermined point. In another example, U.S. Pat. No. 4,321,070, issued to Bede, provides a whistle mounted in the filter. When the filter becomes sufficiently clogged, the air stream is induced to flow through the whistle portion at an increasing rate until an audible signal is produced. U.S. Pat. No. 2,782,747, issued to Alderfer, U.S. Pat. No. 2,753,831, issued to Davies, and PCT Patent Application Serial No. 88/01050 to Horowitz, all also disclose audible change signal devices.
U.S. Pat. No. 2,804,839 discloses a device external to the filter that produces both an audible change signal and visual change signal responsive to an elevated differential pressure drop on either side of the filter. The visual signal is evident external of the filter and does not require removal of the filter itself and visual inspection. However, this device is complicated and expensive relative to the price of the air filter itself and thus does not lend itself to widespread use.
U.S. Pat. No. 3,027,865, issued to Kuntz et al. incorporates a diaphragm that shifts position responsive to a change in the pressure drop across an air filter. The shift in the position of the diaphragm may be used to provide a visual signal or to close an electrical contact in order to provide an electrical signal externally of the filter. U.S. Pat. No. 3,916,817 also discloses a device for providing a visual change signal.
None of these existing air filter change indicators are completely satisfactory. For instance, those that require an elaborate and complicated apparatus to provide a change signal are relatively expensive, and may require setup at installation and periodic maintenance to ensure reliable operation. Those devices that depend on a reduction in airflow to provide an audible signal may be susceptible to clogging by the very contaminants that the air filter is designed to entrap. It is also generally undesirable to require modification to the air circulation system or to the air filter, as is required by many of these approaches.
Moreover, all of these devices provide only one signal. That is, they only indicate when the air filter has reached a predetermined level of lowered performance. It may be desirable in certain situations to have a change indicator that provides a continuous and progressive indication of the performance of the filter.